In next-generation communication systems, active research is being conducted to provide services with various Qualities-of-Services (QoSs) to users. A typical example of the next-generation communication systems is an Institute of the Electrical and Electronics Engineers (IEEE) 802.16 communication system.
The IEEE 802.16 communication system proposes to perform power control using various control schemes such as an open loop power control scheme, a closed loop power control scheme, and an outer loop power control scheme.
The open loop power control scheme, the closed loop power control scheme, and the outer loop power control scheme will be described below.
In the open loop power control scheme, a signal transmitter independently determines the channel state of a signal receiver and performs power control depending on the determined channel state. In other words, the open loop power control scheme controls power based on the reciprocity between an UpLink (UL) channel and a DownLink (DL) channel. For example, the signal transmitter may be a Base Station (BS) and the signal receiver may be a Mobile Station (MS). For convenience of the description, the terms ‘BS’ and ‘MS’ will be used in place of the signal transmitter and the signal receiver.
The term ‘reciprocity between the UL channel and the DL channel’ as used herein means that the UL channel and the DL channel experience similar path losses due to antenna gain based on an antenna pattern, a shading effect caused by geographical features, and multi-path fading, when the positions of MSs against a BS are the same. In other words, in the open loop power control scheme, the BS directly estimates the signal reception quality of an MS based on the reciprocity between the UL and DL channels to calculate required transmit power for signal transmission.
In the closed loop power control scheme, a BS controls the transmit power of an MS based on feedback information, i.e., channel information of the MS, received over a feedback channel from the MS.
In the closed loop power control scheme, the BS receives a pilot signal from the MS, measures a Carrier to Interference and Noise Ratio (CINR) using the received pilot signal, and compares the measured CINR with a predetermined threshold, i.e., a required CINR. If the CINR is less than the required CINR, the BS transmits to the MS a transmit power control message indicating the need for an increase in the transmit power. On the contrary, for the CINR greater than the required CINR, the BS transmits to the MS a transmit power control message indicating the need for a decrease in the transmit power.
The outer loop power control scheme, a scheme for controlling transmit power based on a desired particular performance index (e.g., a target Packet Error Rate (PER)) adaptively changes a power control reference (e.g., a required CINR) according to a channel state in order to keep the target PER constant. The PER indicates an error rate limit for a digital signal, which is required to provide high QoS. Thus, the PER is closely related to the degree of communication satisfaction of a user provided with a service.
As discussed above, power control schemes used for the general communication system include the open loop power control scheme, the closed loop power control scheme, and the outer loop power control scheme. The next-generation communication system performs power control by appropriately combining the open loop power control scheme, the closed loop power control scheme, and the outer loop power control scheme in order to provide services with various QoSs to users.
In the closed loop power control scheme, a BS transmits a transmit power control message to an MS. Upon receiving the transmit power control message, the MS processes the received transmit power control message. Due to the time required for the MS to receive and process the transmit power control message, an error may occur in transmit power, causing degradation in the benefits of power control.
The outer loop power control scheme manages a power control reference depending only on the Modulation and Coding Scheme (MCS) level regardless of the use of a Hybrid Automatic Retransmission reQuest (HARQ) scheme, making it difficult to reflect link performance for each MCS level based on the use and nonuse of the HARQ scheme. Thus, the BS may transmit a traffic burst with the power greater or less than required.